The top three universalizing religions spread through expansion and relocation diffusion. Each has a hearth in Asia: Christianity in Israel, Islam in Saudi Arabia, and Buddhism in India. A hearth is an area where a set of cultural traits and concepts develop. Followers of each religion migrated, preaching the message of the religions to people far beyond the hearth. Christianity spread through relocation diffusion by missionaries and hierarchical diffusion when the Roman Empire made Christianity its official religion. Christianity became the dominant religion in North and South America because of the early explorers and colonizers from Europe were Christians. Islam spread mostly through contagious diffusion by military conquest into North Africa and Western Europe and through Arab traders traveling to Indonesia. Buddhism spread primarily through relocation diffusion by missionaries and was slow to diffuse outside the area of origin.